The present invention relates to systems for forming and transporting particulate materials, and more particularly to a system for forming and feeding meltable or sublimable pellets at low temperatures.
It is commonly known to blast a workpiece with a particulate abrasive that either melts or sublimes at room temperature for cleanly dissipating the abrasive subsequent to its use, thereby avoiding contamination of the workpiece or its environment. The abrasive can be frozen water, typically called "ice", solid carbon dioxide, typically called "dry ice", or combinations comprising one or both of these materials. One well known process for forming the particulate as dry ice is disclosed in U.S. Pat. No. 4,389,820 to Fong et al., wherein liquid CO.sub.2 is dispensed and frozen in a snow chamber, the snow falling into a planetary extruder die mechanism where it is compacted into pellets by being forced through radial holes of a ring-shaped die, the length of the pellets being defined by structure that fractures the material by partially blocking the exit paths from the die.
One problem with pelletizers of this type is that the snow tends to clog in the snow chamber, resulting in interrupted production and/or inconsistent quality of the pellets. This problem has been addressed, for example, in U.S. Pat. No. 3,670,516 to P. Duron et al., which discloses means for mechanically vibrating a cone-shaped snow horn within the snow chamber for shaking off accumulations of the snow therefrom. The Duron et al. patent also discloses a cone-shaped upper portion of the roller carrier, in combination with a leveler strip extending inwardly over the die and rollers for spreading the snow into the path of the rollers. These devices do not fully overcome the problem of clogging, in that the snow is also subject to clogging by accumulating on the inside walls of the snow chamber.
The pellets can be dispensed directly upon formation or they can be stored and/or transported for use upon demand in a hopper or the like. Typically, the pelletizer is permanently or semi-permanently located, the hopper being in a separate mobile unit. In systems wherein the pellets are generated at one location and transferred to a mobile unit for use at a remote work station, the mobile unit typically has an elevated hopper inlet to which the pellets must be lifted from the extruder die mechanism. While mechanisms for lifting and transporting the pellets are known, they contribute undesirably to the expense and complexity of the pelletizer. More importantly, the particles are subject to degradation by subliming, by melting, and by abrasion or pulverization during transport to the workpiece.
Thus there is a need for a particulate formation and delivery system that effectively and reliably feeds the material while avoiding the disadvantages of the prior art.